Application of electronic devices using liquid crystals has not been limited to watches, clocks, thermometers, and other similar devices. Such electronic devices have found wider applications including word processors, laptop computers, and even TV receivers.
N. A. Clark and S. T. Largerwall have made the merits of ferroelectric liquid crystals, in which the liquid crystals themselves have spontaneous polarization, widely known in the industry. Antiferroelectric liquid crystals which are opposite in nature to the above-described ferroelectric liquid crystals have been made widely known in the industry by Chandani and others. These liquid crystals are different in characteristic from generally accepted nematic liquid crystals such as twisted nematic (TN) liquid crystal displays and supertwisted nematic (STN) liquid crystals.
In the exemplary view of FIG. 1, liquid crystal molecules 102 of ferroelectric liquid crystal are oriented in a given direction according to the orientation control of the surface 100 of a substrate. A layered structure 101 is formed between adjacent liquid crystal molecules. Such layered structures are arrayed in a highly organized fashion in three dimensions. Where the cell is thin, the direction of the long axis of each liquid crystal molecule takes two states, i.e., a first state 102 and a second state 103.
A ferroelectric liquid crystal has spontaneous polarization Ps (C/m.sup.2) as indicated by the arrows in FIG. 1. When a voltage is applied to the liquid crystal cell, an electric field is produced perpendicular to the surfaces of the substrates. The spontaneous polarization is directed antiparallel to the direction of the electric field by a torque Ps.E which is the product of the strength of the electric field E (V/m) and the spontaneous polarization Ps. In step with the movement of the spontaneous polarization, the long axis of each liquid crystal molecule is switched between the first state 102 and the second state 103. That is, the state assumed by the long axis can be controlled by the direction of the applied electric field.
Ferroelectric liquid crystal has various features. In particular, the long axis can be quickly switched between the two states, as shown in FIG. 1, by making use of spontaneous polarization. After the application of the electric field, the state can be maintained stably. When observed with polarizer plates, the two states can be distinguished over a wide range of viewing angles. Therefore, it is much expected that ferroelectric liquid crystals will act as liquid crystal materials capable of realizing high-speed viewing screens with high information content.
Usually, a ferroelectric liquid crystal is driven by a simple matrix addressing structure comprising a number of strip-shaped electrodes disposed on a pair of substrates, the liquid crystal being sandwiched between the substrates. When an electric field is applied thereto, this state is stably maintained. That is, information is retained. This feature, which nematic liquid crystals cannot exhibit, is utilized.
This feature is utilized in displays with very high information content, e.g., having 1000.times.1000 pixels or more. These displays are usually driven by a so-called two-field method or four-pulse method. In these drive methods, a small but continuous alternating voltage is applied. Therefore, the waveform induces fluctuations in the optical response of the liquid crystal, thus considerably deteriorating the contrast ratio thereof.
In practice, when a ferroelectric liquid crystal is sandwiched between a pair of substrates and observed with a microscope, the spontaneous polarization is seen to be directed toward either substrate, i.e., uniform orientation, as shown in FIG. 1. In addition, splay orientation is observed, i.e. the spontaneous polarizations of some molecules are directed inward and the spontaneous polarizations of other molecules are directed outward on the surfaces of the substrates. Under this condition, the direction of the long axis of each liquid crystal molecule is bent between the substrates, i.e., in a twisted state. The twisted molecules cannot assume a quenching position. Consequently, contrast is low, the current state cannot be retained, and this orientation state is not practical.
This orientation is in a twisted state when viewed from the long axis of each molecule and in a splay state when viewed from spontaneous polarization. With respect to ferroelectric liquid crystals, both of these mean the same orientation.
Where an image should be displayed with high contrast, a uniform orientation must be always used. Also, each molecule must retain its current state. However, it is difficult to satisfy these two requirements over a wide range of temperatures. As a result, the above-described splay orientation appears.
Presently, it is necessary to drive a ferroelectric liquid crystal capable of maintaining its current state as described above by a separate method and to display images stably without relying only on the simple matrix address driving method. In order to stably drive a liquid crystal material having spontaneous polarization, both a bright state and a dark state should be produced by direct drive, i.e., a voltage is continuously applied to the liquid crystal when an image is being displayed. At this time, the ability to retain the current state is not utilized. The spontaneous polarization and optical response can be completely controlled by the direction of an externally applied electric field.
Taking these facts into account, only one pixel can be used as a simple shutter, although display cannot be performed in a simple matrix panel. This can be employed as a shutter or the like for controlling ON and OFF states for a large amount of light in a projection display. This driving method can include a method of driving a display comprising substrates having pixels incorporating thin-film transistors (TFTs). In any case, the features which cannot be realized by nematic liquid crystals, i.e., fast response and high contrast, can be fully exploited. However, when an image is displayed by a ferroelectric liquid crystal driven by this method, if a liquid crystal material which would conventionally be used to make a simple matrix structure is directly used, then satisfactory results are not obtained.
In particular, a liquid crystal driven by a simple matrix addressing method retains the present state and usually has a small pretilt angle of about 0.degree. to 15.degree., the angle being made between the substrates and the layered structures of the ferroelectric liquid crystal. The angle made between the first and second states assumed by the liquid crystal is often small. This angle, known as the cone angle, is approximately 10to 38.degree..
In order to have a high contrast ratio, the light transmittance must be high in the bright state and low in the dark state. To increase transmittance to its maximum in the bright state, it is necessary for the cone angle to be 45.degree..
Accordingly, the cone angles of the materials for simple matrix structures are too small for directly driven panels which should have high transmittance values. In consequence, materials for use in direct drive must be devised.
In practice, however, some liquid crystals show not only uniform orientation in which spontaneous polarization is directed toward either substrate, but also twisted orientation (i.e., spontaneous polarization is directed inward on the surfaces of both substrates and the long axis of each liquid crystal molecule is bent between the substrates when no electric field is applied). Such a liquid crystal exhibiting the above-described twisted orientation cannot take a quenching position and therefore its contrast is low. If the torque Ps.E is activated by the application of an electric field, every spontaneous polarization is uniformly oriented toward either substrate surface, as shown in FIG. 1.
Referring next to FIG. 2, with respect to antiferroelectric liquid crystals, the direction of the long axis of each liquid crystal molecule assumes a first state 120 and a second state 121 in the same way as the aforementioned ferroelectric liquid crystals. In addition, the direction of the long axis of each antiferroelectric liquid crystal molecule can take a third state 122. When no voltage is applied, the third state is assumed. When a negative voltage is applied, the first state is assumed and when a positive voltage is applied, the second state is assumed.
A clear threshold voltage exists between the third and first states and the third and second states. The presence of these threshold values makes the characteristics of the antiferroelectric liquid crystal differ greatly from those characteristics when the ferroelectric liquid crystal is being driven.
A simple matrix display which drives a liquid crystal by electrodes by making positive use of the features of a ferroelectric or antiferroelectric liquid crystal has been developed. The liquid crystal which is sandwiched between the electrodes is driven by these electrodes machined into strips.
However, it is difficult to develop a high-performance display which activates a liquid crystal material having spontaneous polarization by simple matrix addressing. For this and other reasons, development of a high-performance panel which can display images stably and in which TFTs or metal-insulator-metal (MIM) film nonlinear devices are disposed has been discussed.
Numerous problems which were considered to be difficult to solve have been successfully solved by the use of switching devices described above. In either ferroelectric and antiferroelectric liquid crystals, the assumed one of two or three states is determined only by the direction of the applied electric field. Therefore, it is difficult to vary the gray level by the applied voltage, unlike twisted nematic liquid crystals. Hence, it has been considered that a wide gray scale cannot be readily obtained from ferroelectric and antiferroelectric liquid crystals. As a result, these liquid crystals have not been used in displays required to provide a wide gray scale such as TV displays although they show high-speed switching characteristics and wide viewing angles. Accordingly, there is an urgent need for techniques for realizing displays using ferroelectric and antiferroelectric liquid crystals and having a wide gray scale.
Three approaches are available to meet this requirement. One is to divide each pixel into n parts, and 2.sup.n gray levels are produced by each pixel. In this method, however, the number of pixels is substantially increased by a factor of n. Consequently, the production yield decreases, and the cost is increased.
A second method is to use an analog gray scale which employs TFTs. In particular, a ferroelectric liquid crystal can take both a first state and a second state. The peak value of the voltage applied to the ferroelectric liquid crystal is varied to adjust the ratio of the area of the portions in the first state to the area of the portions in the second state. When a ferroelectric liquid crystal is driven by simple matrix addressing, the electric charge going into and out of the capacitor of each pixel varies constantly and so this second method is impossible to carry out.
However, where TFTs are used, if a gate is turned off after injection of an electric charge, the amount of electric charge going into and out of each pixel electrode via TFTs is zero. Therefore, the liquid crystal can be maintained in a given state. In consequence, a gray scale can be accomplished by varying the area of portions of the liquid crystal in a first state and the area of portions in a second state.
A third method relies on digital gray scale also using TFTs. This makes use of the fact that a ferroelectric liquid crystal assumes only two states, white and black, and responds at a high speed. Various gray levels are obtained by changing the times for which the liquid crystal respectively assumes white and black states. For example, it is assumed that a white state is displayed for 0.2 msec and that a black state is displayed for 0.8 msec. If this process is repeated, then a transmittance of 20% is obtained provided that the observer sees 100% transmittance and 0% transmittance respectively as complete white and black. If the operating frequency is in excess of 60 Hz, then the observer sees no flickering effect. Since a ferroelectric liquid crystal inherently has a high switching speed, if a digital gray scale is used, a display with a wide gray scale can be accomplished.
A digital gray scale is not attained unless a feature of the ferroelectric liquid crystal, i.e. that the response speed is three or four orders of magnitude as high as the response speeds of TN and STN liquid crystals, is fully utilized. Hence, this method makes full use of the high speed of the ferroelectric liquid crystal but requires switching devices such as TFTs or the like.
Where a liquid crystal material having spontaneous polarization is driven by TFTs, a problem not encountered in a nematic liquid crystal driven by TFTs takes place. Obviously, this problem is caused by the fact that switching modes differ according to the liquid crystal material.
The magnitude of spontaneous polarization in liquid crystals normally used lies within the range of 1 to 100 nC/cm.sup.2. Antiferroelectric liquid crystals having spontaneous polarizations of several hundreds of nC/cm.sup.2 are rarely used. This amount of electric charge is that supplied when the liquid crystal is inverted. Inversion does not occur unless at least this amount of electric charge is supplied from the outside. This amount of charge is much larger than the amount of electric charge necessary when driving a nematic liquid crystal. Accordingly, when driving a nematic liquid crystal, it is preferable to use a liquid crystal with a large voltage retention rate. However, this principle cannot be applied to a ferroelectric liquid crystal which is driven by TFTs.
Measurement of voltage-retaining factor as a method for evaluating nematic liquid crystals is described now by referring to FIG. 3. A liquid crystal pixel 134 is connected with a TFT comprising a source 130, a drain 131, and a gate 132. A data signal is supplied to the source 130 through a supply terminal 138. The data signal is routed to the drain 131 in response to a voltage applied to the gate. The signal is then supplied to the pixel electrode. When the gate is off, the resistance between the source and the drain is high and therefore the electric charge supplied to the pixel does not flow in or out via the TFT.
The waveform of this signal is illustrated in FIG. 4. The data signal taking the form of a rectangular wave 143, for example, is applied between the source and drain. A voltage 144 is applied to the drain only when the gate electrode 140 is on. Thus, an ideal voltage 141 stored in a pixel electrode is maintained as a constant voltage without attenuation. On the other hand, a voltage 142 stored in an ordinary liquid crystal pixel is attenuated with time. The effective value of an ideal voltage and the effective value of a voltage in an evaluated liquid crystal is measured. The ratio of the former value to the latter value is referred to as the voltage-retaining factor. Of course, as the voltage-retaining factor approaches 100%, more desirable characteristics are obtained.
Where the voltage-retaining factor is small, the voltage developed across the pixel capacitor varies with time. Since the transmittance of a nematic liquid crystal varies with the applied voltage, if the voltage-retaining factor is small, then the amount of light transmitted through the pixel varies with time. This makes it impossible to have a gray scale with high reproducibility.
The condition obtained when a liquid crystal material having spontaneous polarization is driven by TFTs is described below. The measuring system shown in FIG. 3 is used. The liquid crystal cell 134 may be a ferroelectric liquid crystal having spontaneous polarization or an antiferroelectric liquid crystal. Examples of measurement are illustrated in FIG. 5. Variations in the optical response of the liquid crystal were measured together with the potential at the pixel electrode. As can be seen in FIG. 3, a voltmeter 137 is provided for measuring the potential at the pixel electrode.
When the gate is ON, an electric charge is injected into the pixel electrode. In FIG. 5(1), a constant voltage V.sub.0 152 is supplied. Thereafter, the pixel potential is attenuated and assumes a constant state. At this time, the optical change 155 in the liquid crystal changes from a bright state to a dark state or vice versa. Since this optical change agrees with the decrease in the pixel electrode potential, it follows that inversion of the state of the ferroelectric liquid crystal has consumed the pixel charge. In particular, a charge equal to twice the product of the spontaneous polarization and the electrode area is consumed. As a result, the potential remaining in the pixel is V.sub.rem 153. Thereafter, the pixel voltage and the optical response remain constant. The ferroelectric liquid crystal sufficiently responds with TFTs.
Under this condition, the voltage is changed to 1/2 V.sub.0 156, for example. This state is shown in FIG. 5(2). V.sub.rem 157 drops further, and optical response 158 of the liquid crystal is not sufficient. An intermediate optical position is still assumed.
This phenomenon occurs when the pixel select time is short, as well as when the voltage is reduced. That is, the amount of electric charge injected into the pixel is not sufficient to invert the liquid crystal. Since a ferroelectric liquid crystal is driven by TFTs, using the above-described phenomenon, it is necessary to establish a new method of evaluating a ferroelectric liquid crystal, the method being different from the conventional method of evaluating a voltage-retaining factor. For this purpose, two facts differing essentially from nematic liquid crystals must be understood: (1) when a ferroelectric liquid crystal is switched to a different state, the spontaneous polarization is inverted, thus resulting in a decrease in the liquid crystal potential; and (2) no clear threshold values exist in the inversion.